Closed-loop systems can be further classified as direct return or reverse return.
Direct Return
The direct return system allows piping to be run in the most direct path to optimize piping costs.
The disadvantage is that the flow at each fan coil unit or air handler usually needs to be balanced using a balancing valve.
The length of the water circuit through the supply and return piping to each fan coil or air handler is different in direct return piping. Fan coils close to the pump receive greater flow rate than those further away unless balancing is accomplished.
Open-loop systems such as the condenser water system with a cooling tower are always direct return since individual terminals don’t exist and balancing is relatively simple.
Reverse Return
The reverse return system is piped so that the length of the water circuit through the supply and return piping to each fan coil or air handler is essentially the same. Therefore, pressure drops are basically equal. Buildings such as hotels with multiple identical fan coil units with identical flows are excellent candidates for reverse return systems.
Reverse return has greater pipe lengths and cost. However, the cost of adding a balancing valve for each fan coil using a direct return system could offset the additional costs of the added reverse return piping.
If the individual fan coil or air handling unit water pressure drops are not reasonably close to each other, engineers will often specify balancing valves anyway, regardless of the piping arrangement.
FREQUENTLY ASKED QUESTIONS
In a direct return system, the flow rate at each fan coil unit or air handler is not uniform. Fan coils or air handlers closer to the pump receive a greater flow rate than those further away, unless balancing is accomplished using balancing valves. This can lead to uneven cooling or heating performance across different areas of the building, highlighting the importance of proper balancing in direct return systems.
Balancing valves are used in direct return systems to ensure that the flow rate at each fan coil unit or air handler is consistent, despite the varying lengths of the water circuit. These valves regulate the flow rate by creating a pressure drop, which helps to balance the flow across different branches of the piping system. By installing balancing valves, HVAC designers and engineers can ensure that each fan coil unit or air handler receives the required flow rate, regardless of its distance from the pump.
A reverse return system is piped so that the length of the water circuit through the supply and return piping to each fan coil or air handler is essentially the same. This design approach eliminates the need for balancing valves, as the flow rate at each fan coil unit or air handler is inherently balanced due to the equal piping lengths. Reverse return systems can provide more even cooling or heating performance across different areas of the building, but may require more complex piping designs and higher upfront costs.
Yes, direct return systems can be used in combination with other piping configurations, such as loop systems. In fact, many modern HVAC systems incorporate a hybrid approach, where direct return systems are used for certain branches of the piping network, while loop systems are used for others. This hybrid approach can help to optimize piping costs, simplify system design, and improve overall system performance.
Direct return systems are often preferred in applications where piping costs are a significant concern, such as in large commercial or industrial buildings with extensive piping runs. They are also commonly used in retrofit projects where existing piping infrastructure needs to be reused or modified. Additionally, direct return systems may be preferred in applications where simplicity of design and installation is a priority, such as in smaller buildings or in areas with limited access.
